A velocity-measuring correlation SONAR or RADAR functions by correlating a first echo of a first transmitted pulse received at a first receiver with a first echo from a later-transmitted pulse received at a second receiver. Maximum correlation occurs when the ray path of the initial transmission (e.g., from the transmitter to the ocean floor and back to the first receiver, etc.) is equal to the ray path of the second transmission. The velocity of the vessel is calculated based upon the distance traveled by the vessel between the transmission and reception of the pulses. See, for example, U.S. Pat. No. 4,244,026 to Dickey and U.S. Pat. No. 5,315,562 to Bradley et al.
Examples of velocity-measurement SONARs are spatial correlation SONAR and temporal correlation SONAR, which rely on selecting a maximum correlation between hydrophones in the case of spatial correlation or pulses in the case of temporal correlation.
Spatial correlation SONAR calculates the velocity of a vessel by transmitting two or more pulses towards the ocean bottom, detecting echoes of the pulses on a planar two-dimensional array of hydrophones, determining which two hydrophones in the array correlate the best, and dividing the distance between those hydrophones by twice the time differential between the pulses. Peak correlation might take place between hydrophones, in which case an interpolation scheme is used. Since the later echo is received on a number of receivers located in the two-dimensional array, a velocity solution can be estimated for more than a single direction (e.g., forward and athwart components, etc.).
Velocity estimates from correlation SONARs are subject to accuracy degradation due to various random and bias errors, and accuracy is particularly degraded under extended operational conditions, such as shallow ocean bottom depths and high ship's speed.
The prior art has attempted to improve the accuracy of velocity-measuring spatial correlation SONARs via a variety of techniques, including (1) processing more data via increased processing throughput, (2) altering the physical dimensions of the hydrophone array, (3) altering the number of hydrophones in the array, (4) modifications to transmit pulses and/or pulse patterns, and (5) applying temporal correlation SONAR techniques, among others.
A further approach to improving the accuracy of velocity-measuring correlations SONARs was patented by the present inventors in U.S. Pat. No. 7,295,492. The method disclosed therein develops multiple velocity estimates using different receiver pairs. The velocity estimates are filtered to yield a single improved velocity solution.
It would be beneficial to improve the accuracy of velocity-measuring spatial correlation SONARs via a method that (1) does not physically alter an existing receiver array, (2) is less-processing intensive than prior art enhanced-processing approaches, and (3) provides particularly improved accuracy under extended operational conditions.